The transfer of small, dry toner particles, for example, toner particles of less than 5 microns in size from a photoconductor or other image member to a receiving sheet is extremely challenging. Studies on the forces which move small particles indicate that as the particle becomes smaller the effect of an electrostatic field is less on a particle compared to the effect of ordinary adhesive forces. This has made conventional transfer using an electrostatic field relatively ineffective in transferring such small particles. See, U.S. Pat. No. 5,084,735, Rimai et al, issued Jan. 28, 1992 and U.S. Pat. No. 4,737,433, Rimai et at.
U.S. Pat. No. 4,968,578, Light et at, issued Nov. 6, 1990; U.S. Pat. No. 4,927,727, Rimai et al, issued May 22, 1990; and U.S. Pat. No. 5,021,835, Johnson, issued Jun. 4, 1991, all describe a heat assisted toner image transfer method particularly usable with small particles. Two or more single color images are transferred in registration from an image member to a receiving sheet by heating the receiving sheet to an elevated temperature. The temperature of the receiving sheet is sufficiently high that the toner sticks to the receiving sheet and to itself. Preferably, the receiving sheet is heated from inside a transfer drum to which it is secured. The transfer drum and image member form a pressure nip with the combination of heat and pressure transferring the image. This method is particularly useful in transferring extremely small, dry toner particles, for example, toner particles having a mean particle diameter of 5 microns or less.
In a preferred form of the heat assisted transfer described in these references a receiving sheet having a heat-softenable outer layer is used. The receiving sheet is heated to a temperature which softens the outer layer and the first layer or layers of the toner images partially embed themselves in the heat-softened layer to assist in transfer of the first image or so. Further layers of toner from subsequent images or dense portions of the first image attach themselves to toner particles that are partially embedded. With extremely small, dry toner particles this method provides extremely efficient transfer with excellent resolution.
Although heat assisted transfer to a heat-softened layer provides the most efficient and highest resolution transfer of very small toner particles known in the prior art, it is not without problems. Depending somewhat on the materials, relatively high pressures are desirable, for example, pressures of up to 500 pounds per square inch and higher. Heating is accomplished generally through the receiving sheet. Even if the receiving sheet is carried on a metallic drum, it is somewhat difficult to maintain the temperature of the thermoplastic layer within limits that will sinter the toner without overheating the image member or blistering the receiving sheet. Overheating of the image member can cause damage to it, including a reduction of its ability to hold a charge. Overheating of the toner image can cause sticking to the image member and/or spreading of the image. It is known to provide a heating element inside a photoconductive drum which heats the drum to an elevated but safe temperature for the image member and thereby requires less heating from the transfer member. Even with this useful approach, temperature control at transfer is difficult with a receiving sheet receiving the images from a photoconductor.
Especially in transferring a series of single color toner images to form a multicolor toner image, the layers of toner pile up above the level of the receiving sheet even when substantial pressure is used in transfer and with a heat-softened layer receiving the toner. This results in an unacceptable relief image corresponding generally to the optical density of the image. U.S. Pat. No. 5,023,038 to Aslam et at, issued Jun. 11, 1991 and U.S. Pat. No. 5,089,363 to Rimai et al describe a method of fixing such toner images to a receiving sheet which receiving sheet has an outer heat-softenable thermoplastic layer. The relief image is substantially reduced, the image is more permanently fixed and gloss can be increased by bringing the image into contact with a ferrotyping surface under conditions of heat and pressure which cause the image to be further embedded in the thermoplastic layer. The ferrotyping surface is smooth and hard and has good release characteristics. For example, it can be made of nickel, stainless steel or other metals, with or without surface treating with silicones or the like. For highest quality imaging, the ferrotyping surface is left in contact with the image until the image and heat-softened layer have cooled below their glass transition temperatures, at which point it is separated without offset.
The use of endless belts generally to fix regular toner images directly to paper, transparency stock, or the like has been known for many years; see, for example, U.S. Pat. No. 3,948,215; European Applications 0301585 and 0295901. Typically, in all of the above fixing processes the toner image is left in contact with the web until the image is cooled below the glass transition temperature of the toner, at which point the receiving sheet can be separated without offset. Preventing offset by cooling in contact with the web eliminates the need for offset preventing liquids which have a degrading effect on a high-quality image.
A problem in using a web system, especially an endless belt system, in a productive image forming apparatus is associated with the time required for the belt and image to cool while maintained in contact. If the fixing device is slowed down to below the speed of the transfer station to allow cooling, then the mismatch of speeds between the transfer station and the fixing device must be accommodated. In general, this requires either a full frame distance in the in-track direction between the transfer station or drum and the fixing device, or loop or other mechanism for absorbing the difference in speeds.
Belt fixing devices create other non-trivial problems. For example, belt tracking must be controlled. The belts are expensive and difficult to replace. If the belt has a seam the timing of the apparatus must be controlled to prevent the seam appearing in the middle of an image. The convenient availability of different textures is accomplished generally by exchanging belts, a task which is time consuming and especially difficult if the apparatus is hot. The belt has very limited room inside it for cooling structure.
An intermediate transfer member (sometimes herein called an "intermediate") has been used in both single color electrophotography and multicolor electrophotography. For example, U.S. Pat. No. 4,931,839, shows the use of an intermediate conductivity intermediate web to accumulate several single color toner images by separate electrostatic transfer from a photoconductive web. The multicolor image formed on the intermediate is electrostatically transferred to a receiving sheet and later fed to a separate fixing station.
A number of references show single color transfer to an intermediate and then a combined step including simultaneous fixing and transfer under heat and pressure from the intermediate to a receiving sheet. See, for example, U.S. Pat. No. 4,657,373; U.S. Pat. No. 4,068,937; U.S. Pat. No. 3,893,761; U.S. Pat. No. 4,453,820; and U.S. Pat. No. 4,542,978. In each of these references, the intermediate has a silicone rubber or other compliant surface which is used because of its affinity to toner at the first transfer step. At or before the second transfer step the image and, in some instances, the receiving sheet are preheated so that transfer and fusing can be accomplished in a single step. The intermediate is generally cooled before it returns to the original image member to pick up additional images for fear of damage to a photoconductor or other sensitive portion of the original image member.
U.S. Pat. Nos. 4,588,279; 4,455,079; and 4,518,976 maintain the receiving sheet in contact with the intermediate until the image has cooled before separation. As in the previous references, silicone rubber is generally used for the intermediate in order to accomplish pressure transfer without heat at the fast or original transfer.
U.S. Pat. No. 4,910,558 shows an intermediate drum which is internally heated and covered with compressible silicone rubber. A receiving sheet is preheated to the temperature of boiling water and pressure is used at both transfers.
U.S. Pat. No. 4,912,514 shows an intermediate web with a conductive base and a fluoride coating with separate rapid heating components opposite the original transfer from a photoconductive drum and opposite a combination transfer-fusing position where the single image is transferred to and fused to a receiving sheet. The first transfer is said in the reference to involve fusing the toner on a photosensitive drum until it transfers to and is temporarily fixed on the surface of the intermediate.
U.S. Pat. No. 4,531,825 shows an intermediate roller having a heat conductive core with a silicone or fluoride resin coating. The original image member has a soft backing providing a larger nip for the first transfer and the transferred image is fused and transferred to a preheated receiving sheet in a single step.
U.S. Pat. No. 4,992,833 shows an intermediate sheet or web to which a single toner image is transferred by means not described. After the transfer the image is fused to the intermediate and kept warm until overlaid with a receiving sheet. The receiving sheet and intermediate are cooled together before separation. The intermediate is cleaned for reuse.
U.S. patent application No. 612,948, filed Nov. 13, 1990 (CIP of U.S. patent application No. 448,487, now abandoned) to Y. Ng discloses using thermally assisted transfer for three or four small particle color toner transfers to an intermediate and a combination transfer and fuse of the resulting multicolor image to a receiving sheet.
Japanese Kokai 1-179181; published Jul. 17, 1989, shows a combination of heat and electric field used to transfer a toner image to a receiving sheet carried by either a drum or belt. The image is fixed while the receiving sheet is still on the drum or belt by pressure contact with a heated roller.